This invention relates to a flyback converter which is usable as a kind of DC--DC converters.
Such a flyback converter comprises a flyback transformer and a current control circuit which will presently be described. The flyback transformer has an inductance value and processes an exciting current into an excited current under influence of the inductance value. The inductance value is constant while the flyback converter carries out a predetermined operation. Depending on the circumstances, the inductance value will be called a general inductance value.
The current control circuit is connected to the flyback transformer and controls the exciting current with reference to the excited current so that the exciting current is intermittently supplied to the flyback transformer. As a result, the exciting current is supplied to the flyback transformer as a succession of pulses. Each of the pulses has a pulse width corresponding to a time interval which will hereafter be called an on-period. The flyback transformer produces the excited current when the exciting current is absent.
In the manner known in the art, the on-period varies in response to magnitude of a load which is driven by the flyback converter. For example, the on-period is long when the load is relatively large. The on-period is short when the load is relatively small.
The on-period has a minimum value determined in accordance with a peripheral circuit which is related to the flyback converter. This means that it is impossible in the flyback converter to carry out the predetermined operation in a particular case where the load is very small.
In order to enable the predetermined operation in this particular case, a dummy resistor is added to the flyback converter in parallel with the load. However, the dummy resistor results in the predetermined operation at a reduced efficiency.